Platesetter

ABSTRACT

Apparatus for scanning a light-beam across a sheet mounted on a flat bed, the apparatus including a bridge that extends across the bed from one side to the other and is supported at each end on carriage guided along a respective one of said sides of the bed, a scanning head that is su ported on the bridge, drive devices at each end of the bridge to drive a respective carriage, location devices at each end of the bridge to monitor the position of each carriage along a respective one of said sides, and control device to control operation of each drive devices in accordance with information of the position of the respective carriage driven by the drive devices, determined by the respective location devices, so that both ends of the bridge are moved simultaneously in timed relation to one another.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to apparatus for scanning a modulated light-beamacross a photosensitive surface, and especially apparatus commonly knownas platesetters, in which an image is scanned directly onto thephotosensitive surface of a printing plate.

2. Description of the Background

The plates used in platesetters are generally made of aluminium sheetand are supplied in different sizes to suit different size printingjobs. In use, each plate is mounted on a scanning bed of theplatesetter, which is typically a fixed concave bed to suit the curvedfocal plane of a particular rotary mirror, light-beam scanning system.The aluminium plates are thin enough to be flexible so as to conform tothe concave shape of the bed, and are held in position by mechanicalfastenings and/or pneumatic suction means.

The quality of the print image produced is principally dependent uponthe accuracy of the light-beam scanning system, and this is why curvedbed platesetters have been preferred. The productivity of a platesetteris dependent upon the speed of operation of the scanning system.However, the time taken in loading and removing plates from the bed alsohas a major effect on productivity and also effects quality adversely ifthe plates are not secured adequately, especially at the edges wheretheir natural resilience can cause them to lift away from the bed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improvedplatesetter, in particular, a platesetter with an improved platehandling system. Another object is to provide a platesetter which isadapted to serve the alternative function of scanning a modulatedlight-beam across a photosensitive film which is used in a separateprocess to produce a printing plate.

According to a first aspect, the invention consists in a platesetterhaving a flat bed, a light-beam scanning system mounted above the bed soas to scan a photosensitive plate on the bed, a store for photosensitiveplates located beneath the bed, and feed means for feedingphotosensitive plates from the store to the bed for scanning.

Preferably, the plates are stored upright in one or more cassettesbeneath the bed, and the feed means selects plates as required from thecassettes and feeds each to a feed station along one side of the bed,from where the plate is fed horizontally onto the bed, preferably edgefirst with the leading edge generally normal to the feed direction.

It will be appreciated that by locating the plate store beneath the bed,a compact and convenient arrangement is achieved which readilyfacilitates loading of plates into the store from one side of theapparatus extending generally parallel to the feed direction.

According to a second aspect, the invention consists in a printsetterhaving a flat bed, a light-beam scanning system mounted to scan aphotosensitive plate on the bed, a plate feed station located along afirst side of the bed to feed photosensitive plates one at a time ontothe bed in a first feed direction, and a film feed station located alonga second side of the bed adjacent said first side to feed photosensitivefilm sheet onto the bed in a second feed direction transversely of sadfirst feed direction.

Preferably, the plates are stored upright in cassettes beneath the bed,and feed means selects plates and feeds them to the feed station. Thecassettes are loaded beneath the bed along the same side as the filmfeed station so that the handling of the photosensitive mediaconveniently takes place in a common work area which may be incorporatedin a "darkroom".

Preferably, the plates and film sheet are removed from the bed in saidsecond feed direction so that they can be further handled or processedin the same work area.

According to a third aspect, the invention consists in apparatus forscanning a modulated light-beam across a photosensitive sheet, such as aphotosensitive printing plate, comprising a flat bed to support saidsheet, a light-beam scanning system to scan a photosensitive sheetmounted on said bed, and sheet location means comprising air ductsadapted to supply a layer of air to the bed on which the sheet can"float", and one or more stop members located on the bed in relation toan inclination of the bed from the horizontal so that a sheet "floating"on the bed slides under the action of gravitational force against thestop. Preferably, stop members are located to support adjacent lowestedges of said sheet so that it is located in mutually orthogonaldirections on said bed.

This arrangement of pneumat ic support and gravitational self-locationof a sheet is simple to provide, and is rapid and effective inoperation, especially with photosensitive printing plates that arerelatively rigid and weighty.

According to a fourth aspect, the invention consists in apparatus forscanning a modulated light-beam across a photosensitive sheet comprisinga flat bed to support said sheet, a light-beam scanning system includinga scanning head mounted above the bed so as to scan a sheet on the bed,and sheet pick-up means, such as pneumatic suction means, mounted on thehead to contact the upper surface of the sheet and hold it so that itcan be moved by the head to be located in position on the bed forsubsequent scanning.

The dual use of the scanning head for light beam scanning and sheetlocation serves to simplify the design of the apparatus.

If necessary, the aforesaid pneumatic support and gravitationalself-location arrangement for a sheet can be used in combination withthe scanning head sheet pick-up means so as to achieve final location ofa sheet once released by the pick-up means onto the bed. For example.different size sheets may each require to be located differently inrelation to the stop members by the scanning head pick-up means.

According to a fifth aspect the invention consists in apparatus forscanning a light-beam across a sheet mounted on a flat bed, theapparatus comprising a bridge that extends across the bed from one sideto the other and is supported at each end on a carriage guided along arespective one of said sides of the bed, a scanning head that issupported on the bridge, drive means at each end of the bridge to drivea respective carriage, location means at each end of the bridge tomonitor the position of each carriage along a respective one of saidsides, and control means to control operation of each drive means inaccordance with information of the position of the respective carriagedriven by the drive means, as determined by the respective locationmeans, so that both ends of the bridge are moved simultaneously in timedrelation to one another. By these means, the bridge can be maintainedorthogonal to the axis along which it is scanned across the bed, whilethe scanning head is moved the length of the bridge so as to scan thebed along a second axis perpendicular to said first axis.

The drive means preferably comprise electric linear motors.

Preferably, the ends of the bridge are pivotally connected to thecarriages so as to accommodate slight differential movements of thecarriages without impairing the movement of either carriage along saidsides. The location means, preferably takes the form of an opticalgrating along each side and is positioned as close as possible to thesepivotal connections to minimise tilting errors in the carriage positionmeasurements.

Preferably, the centre of gravity of the scanning head is aligned withthe respective pivotal connections at the ends of the bridge so thatacceleration of the scanning head does not apply a torque to the bridgethat would adversely effect the positions of the carriages at each end.

Preferably, drive means, such as an electric linear motor, and locationmeans, such as an optical grating, are provided on the bridge to controllongitudinal movement of the scanning head along the bridge.

The scanning head preferably scans the light-beam repeatedly across ashort distance and is advanced between individual scans so thatsuccessive lines are scanned across the direction of advance of thescanning head along the bridge, the light-beam thereby covering a bandor swathe in a single traverse across a sheet being scanned. Successivetraverses of the scanning head are offset each by the width of a swathefrom the last swathe as the bridge is indexed across the bed so th t thewhole of the sheet is scanned by the head.

According to a sixth aspect, the invention consists in a scanning systemin which a scanning head is located along a scanning axis by a locationsystem comprising an optical sensor and a grating which move relative toone another, the sensor producing sine and cosine voltage output signalscorresponding to the spacing of lines ruled at regular intervals alongthe grating, the system further comprising a processor that processesthese signals to compensate for variations in the distance between thesensor and the grating, the processor serving to determine theinstantaneous value of the root of the sum of the squares of the outputsignals, and divide each of the output signals by this root-mean-squarevalue before processing the output signals to determine the position ofthe scanning head.

According to a seventh aspect, the invention consists of an apparatusfor scanning a modulated light-beam across a photosensitive sheetcomprising a flat bed to support said sheet, a light-beam scanningsystem including a scanning head mounted above the bed so as to scan asheet on the bed, and sheet pick-up means mounted on the head to contactthe upper surface of the sheet and hold it so that it can be moved bythe head to be located in position on the bed for subsequent scanning.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIG. 1 shows a schematic plan view of a platesetter according to theinvention;

FIG. 2 shows a side elevation of the side A of the platesetter of FIG.1;

FIG. 3 shows a side elevation of the side D of the platesetter of FIG.1;

FIG. 4 shows a detail of one end of the scanning bridge from FIG. 3;

FIG. 5 shows a plan view of that end of the scanning bridge shown inFIG. 3; and

FIG. 6 shows a section through the scanning head shown in FIG. 1.

FIG. 7 shows a plan view of the pinch along side C of the platesetter.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The illustrated platesetter consists of a rectangular frame 1 which hasfour upright sides A,B,C,D and supports a float rectangular bed 2 at itstop to receive photosensitive sheet material to be scanned. A light-beamscanning mechanism is also supported on top of the frame 1, and consistsof a scanning bridge 3 that extends over the bed 2 from one side A tothe other side B and is supported at each end on a carriage 4 that runson a guide 5 along a respective side A,B of the bed. The bridge 3 istherefore guided on these parallel guides 5 so as to traverse the be 2on a line X parallel to the guides. A scanning head 6 is mounted on thebridge 3 and is guided along the length of the bridge so as to scan thebed along a line Y substantial parallel to the sides C and D andperpendicular to the line of movement X of the bridge. A laserlight-beam source 7 is mounted on the head 6 and is directed at the bedso that it can be scanned across a sheet of photosensitive material onthe bed by controlled movements of the head and bridge along theirrespective orthogonal lines of movement X and Y.

The photosensitive sheet material may take the form of rectangularflexible aluminium plates 8 which when exposed and processed form aprinting plate. These plates are stored upright in a store 9 beneath thebed with a plate feed mechanism 10 that feeds plates one at a time asrequired, onto the bed through a feed slot 11 along one side D of thebed beneath the bridge start position. Each plate fed through the slot11 moves along the line of movement X of the bridge with its leadingedge extending generally parallel to the bridge and sides C and D.

The store 9 consists of a set of cassette holders 12 that extend betweenthe sides A,B and are supported together on a lower mobile frame 13 thatis moveable by a drive mechanism 14 along a line between the two sidesC,D. Each cassette holder 12 is adapted to support an upright cassette15 in which plates 8 of a particular size are stored. These cassettes 15are loaded into the holders 12 from the side A. A roller shutter door 16closes the front of each cassette that faces the side D, and has to berolled downwards to open the cassette for removal of a plate.

A plate 8 of a particular size is selected for feeding to the bed 2 bymoving the mobile frame 13 with the drive mechanism 14 until thecassette containing the required plate is located in a feed positionbeneath a belt feed mechanism 17. A door opening mechanism (not shown)then engages lugs 18 on the door 16 of the cassette to roll itdownwards. A plate gripping mechanism 19 consisting of a set ofpneumatic suction pads 20, operates under the control of a pair of fluidactuated rams 21,22 at each end of a lateral support 23 carrying thesuction pads 20, so that the pads 20 are moved downwards in front of thecassette to engage the face of the forward-most plate within thecassette 15. Suction causes the pads 20 to grip the plate 8 so that itis tilted forwards at the top out of the cassette 15 as the horizontalrams 22 move the pads 20 forwards. The vertical rams 21 then lift thewhole plate 8 so as to feed its upper edge into the pinch between afluid actuated pinch roller 24 and a lower roller 25 of the belt feed17. Once gripped by the pinch roller 24, the suction applied to the pads20 is released and the belt feed 17 operates to feed the plate onto thebed 2.

A further set of suction pads 26 operate in synchronism with the suctionpads 20 and serve to grip a protective sheet interleaved between plates8 in the cassette, and removes this horizontally across a cover 27 overthe cassettes 15 towards side C, where it is deposited in a waste bin28. The pads 26 are mounted at the end of individual fingers 29 whichare pivotally connected to a closed loop drive mechanism 30 extendingover the cover 27 and serving to move the fingers 29 between the beltconveyor 17 and bin 27 and back again along a set feed path. At that endof the feed path adjacent to the belt conveyor 17, the fingers 29 pivotdownwards so that the suction pads 26 at their ends engage theprotective sheet in the cassette. Continued operation of the drivemechanism 30 then serves to withdraw the fingers 29 from the cassette asthey make a return movement along the feed path.

The belt conveyor 17 consists of a continuous belt feed 31 and a set offeed rollers 32 which grip and feed each plate 8 between them. The beltfeed 31 consists of an infeed belt 33 and an outfeed belt 34, bothlinked together on a common intermediate roller 35. The infeed belt runsover the pinch roller 24 at the input end, and the outfeed belt 34 isdriven by a motor 36 which in turn drives the infeed belt 33 via theintermediate roller 35. The feed rollers 32 are spaced along an arcuatefeed path from the lower roller 25 to an upper roller 37 so that plates8 follow this path as they are fed by the belts 31,33 and exit at theupper end on a tangential path towards the feed slot 11 formed betweenthe edge of the bed 2 and the bridge 3. As shown in FIG. 3, five similarbelt feed units 31 are aligned horizontally across the frame between thesides A and B, but a single central belt feed unit 31 could be used inan alternative embodiment.

The flexibility of each plate 8 allows it to flex as it is fed betweenthe belts 33,34 and rollers 35 but it is still stiff enough to extendunsupported from the belt feed unit 31 through the feed slot 11. Oncethe belt feed unit 31 has delivered the plate fully, the leading sectionlies on the bed 2, but the trailing section still extends rearwardsthrough the feed slot 11 beneath the bridge 3. A pair of pneumaticsuction pads 38 on the underside of the scanning head 6 then serve togrip the plate 8 so that movement of the head can be used to deliver theplate 8 to an initial delivery location on the bed 2, with adjacentedges of the plate generally parallel to the sides B and C.

Final location of the plate 8 on the bed 2 is achieved by floating theplate on a cushion of air produced by air delivery ducts 39 distributedacross the upper face of the bed. The bed 2 is inclined slightly fromthe horizontal along both lines X and Y, the bed being lower along thesides B and C so that the plate floats under the effect of gravitytowards these sides B and C and engages respective stops 40 along thesesides. The air supply to the ducts 39 is then switched off, and suctionapplied so as to grip the plate 8 and hold it in this final location forscanning.

Movement of the bridge 3 across the bed 2 in the X direction is effectedby drive means at each end which acts through the carriage 4 on whichthe end of the bridge is supported. The drive means comprises anelectric linear motor 41, shown in more detail in FIGS. 4 and 5, whichcomprises an annular winding 42 clamped by a bracket 43 to an outerupright flange 44 of the carriage 4, and a magnetic rod 45 that extendsthrough the centre of the winding 42 along a respective side A,B of thebed, and is secured to the frame by brackets 46 at each. Energisation ofthe winding 42 produces an axial force on the winding in the normalmanner of a linear motor, and causes the carriage to move along theguide 5.

A slide 47 connects the carriage 4 to the guide 5 and consists of are-circulating ball-bearing type slide having a C-shaped cross-sectionto match the cross-section of the guide, which has retaining channels 48along each side. The slide 47 is connected to the underside of ahorizontal flange 49 of the carriage which projects inwards from theouter upright flange 44. The bridge 3 is connected to the upper surfaceof the flange 49 by three shoulder bolts 50,51,52 which extend downwards through holes in a lower wall 53 of the bridge and are threadedinto the flange 49. Each bolt is provided with a bush 54 within the holein the wall 53 so that the bolts secure the bridge and carriage togethervertically without applying a load to the interface between thesecomponents. This is arranged in order to allow a limited pivotingmovement of the bridge 3 about one of the bolts 51. The bush 54 aroundthis bolt 51 is rigid and serves as an annular bearing. However, thebushes 54 around the other two bolts 52,53 are resilient so that theywill accommodate the required limited pivoting movement of the bridge.The pivot bolt 51 is aligned vertically with the centre-line of theguide 5, and the other bolts 52,53 are located outboard of the bolt 51.

The limited pivotal movement of each end of the bridge 3 on therespective carriages 4 is provided to allow each end of the bridge 3 tobe moved independently by the respective linear motors 41 so that thebridge can be positively maintained orthogonal to the line of movement X(i.e. aligned on the line Y).

Control of the position of each end of the bridge 3 is effected by anoptical sensor 55 on the carriage 4 which scans an optical grating 56that extends along a respective side A,B of the bed 2. The sensor 55 isa known type of encoder which works with a grating 56 to produceanalogue sine and cosine output voltages at a pitch corresponding to thepitch of the lines ruled on the grating, typically 20 microns. Ascanning control system receives these analogue signals and processesthem into digital form to determine the approximate position of thecarriage in terms of the zero cross over points, which occur atintervals of 5 microns with a 20 microns grating pitch. Furthermore, thecontrol system also incorporates analogue servo-control in which theamplitude of the output voltage signals from the encoder 55 can be usedto determine the final position of the bridge to a higher accuracy.

As shown in FIG. 4, the encoder 55 is fixed to the underside of thecarriage 4 in the angle between the flanges 44,49, and the grating 56 isattached to the adjacent surface 57 of a rebated edge of the bed 2 towhich the guide 5 is also connected. The grating 56 is located outboardof the guide 5, but close to it so that the position reading given bythe encoder and grating corresponds as closely as possible to theposition of the pivotal connection 51 of the bridge to the carriage.

The accuracy of the position readings given by the encoder and gratingis dependent upon the amplitude of the sine and cosine output signalsfrom the encoder 55, and these are therefore sensitive to variations inthe Spacing of the encoder 55 from the grating 56 as the two moverelative to one another. Typically, such variations can cause theoutputs from the encoder to vary by a factor of 4. In order tocounteract such effects, the output signals are subject to compensationprocessing be fore they are used to determine a position. Thiscompensation process makes use of the trigonometric identity:

    sine.sup.2 +cos.sup.2 =1

Assuming that the sine and cosine signals from the encoder are subjectto a spacing error factor k, this factor k can be determined byroot-mean-square processing using a standard log-antilog circuit afterthe signals have been rectified. That is:

    [(k sine).sup.2 +(k cos).sup.2 ].sup.1/2 =k(sine.sup.2 +cos.sup.2).sup.1/2 =k

The sine and cosine output signals are then divided by the factor k toproduce compensated outputs for position measurement.

Movement of the scanning head 6 along the bridge 3 is also effected byan electric linear motor 58, which is shown in cross-section in FIG. 6.The bridge 3 is a box-section member and is formed with al open recess59 along one side in which is mounted a magnetic rod 60 so as to extendthe length of the recess. The head 6 consists of a horizontal platform61 mounted via a pair of slides 62 on a pair of guides 63 extending thelength of the bridge, these slides 62 and guides 63 being similar to theequivalent components 47 and 5 used with the carriages 4. A flange 64extends downwards from the platform 61 alongside the opening of therecess 59, and an annular winding 65 is clamped to the inside face ofthe flange 64 by a bracket 66, and extends into the recess 59 andsurrounds the magnetic rod 60. The winding 65 and rod 60 togetherconstitute the linear motor 58 that moves the platform along the bridge3 in the Y direction across the bed 2.

Control of the position of the scanning head 6 along the bridge iseffected by an optical sensor 69 mounted at the lower end of the flange64 so as to scan an optical grating 70 on the adjacent side of thebridge 3. The sensor 69 and grating 70 are similar to the equivalentcomponents 55 and 56 that control the position of the bridge 3 in the Xdirection, as described above, and together they produce outer t signalsthat control the linear motor 58 so as to position the head 6 to 5micron intervals along the bridge corresponding to the line spacing ofthe grating.

The laser light-beam source 7 is mounted on top of the platform 61together with a lens system 67 that directs a laser beam downwardsalongside the flange 64 onto the bed.

The suction pads 38 on the head that pick-up plates on the bed, asdescribed above, are mounted on the outside of he flange 64 viarespective rams 68, which serve to raise and lower the pads 38 asnecessary.

The lens system 67 includes canning means that scans the beam across apredetermined short width of the bed 2 along the line X so that a bandor swathe is scanned by the beam as it is indexed along the bridge andmakes successive scans at intervals of 5 microns. Thus, a plate 8located on the bed 2 is scanned in successive swathes across it in the Ydirection.

Once a plate 8 has been scanned to expose it and form a print imagethereon, it is removed from the bed by a pick-up mechanism 71 along theside B which moves over the bed, picks-up the plate 8 by pneumaticsuction and removes the plate along the line Y.

A punch set 72 is mounted along side C and moves over the bed to punchregistration holes along the adjacent side of the plate 8 after it isgripped by suction on the bed 2 and before it is scanned. The punch setconsists of a number of punch units 75 spaced along the side C (five asshown in FIG. 7) and mounted together on a common support member 76which is guided by slides 77 relative to the bed 2 so as to advancetowards and retract from the adjacent edge of the bed under the controlof a central fluid actuator 78. The adjacent edge of the bed is providedwith a set of notches 79 to receive the punch units 75 when they areadvanced towards the bed to punch register holes in a plate 8 whichoverlies the notches 79. The punch units 75 are opera ed together topunch a sheet through a common cam shaft 80 operated by a fluid actuator81.

As shown in FIG. 1, the steps 40 that locate the sheet 8 on the bed 2along side C are located on the bed 2, bit in an alternative embodimentof the invention these stops 40 can be located on the punch units 75 andthe punch units engaged with the edge of the bed to receive and locatethe sheet 8 when it is loaded onto the bed. The plate 8 is then held inposition by suction and the punches punch the register holes. The punchset is then withdrawn from the side of the bed to allow the plate to bescanned.

As an alternative to the scanning of the aluminium plates 8 to formprinting images on them, photosensitive film material lay be mounted onthe bed and scanned in a similar manner. The film is stored in roll formin a film store 73 along side A of the bed and is force fed out of thebed in direction Y by a feed mechanism in the store. The suction pads 38on the head 6 may be used to grip the film and move it in the Ydirection. A guillotine in the store 73 cuts the film to length, andonce located in the final scan position, the film is held by suctionapplied to the ducts 39 on the bed. The pick-up mechanism 71 thenremoves the film from the side B once it has been exposed.

It will be appreciated that all of the control functions of theapparatus including the plate and film feed mechanisms, the beamscanning system and the laser beam modulation system, are convenientlyall centralised in a processor 74 which incorporates operator controlsto set up the apparatus for any particular use.

What is claimed is:
 1. Apparatus for scanning a light-beam across asheet mounted on a flat bed, the apparatus comprising a bridge thatextends across the bed from one side to the other and is supported ateach end on a carriage guided along a respective one of said sides ofthe bed, a scanning head that is supported on the bridge and moveablealong the length of the bridge so as to scan along a directionperpendicular to the motion of the bridge, drive means at each end ofthe bridge to drive a respective carriage, location means at each end ofthe bridge to monitor the position of each carriage along a respectiveone of said sides, and control means to control operation of each drivemeans in accordance with information of the position of the respectivecarriage driven by the drive means, as determined by the respectivelocation means, so that both ends of the bridge are moved simultaneouslyin relation to one another.
 2. Apparatus as claimed in claim 1 in whichthe drive means comprise electric linear motors.
 3. Apparatus as claimedin claim 1 in which the ends of the bridge are pivotally connected tothe carriages so as to accommodate slight differential movements of thecarriages without impairing the movement of either carriage along saidsides.
 4. Apparatus as claimed in claim 3 in which the location meanstakes the form of an optical gratin g along each side and is positionedclosely adjacent to these pivotal connections to minimise tilting errorsin the carriage position measurements.
 5. Apparatus as claimed in claim4 in which the centre of gravity of the scanning head is aligned withthe respective pivotal connections at the ends of the bridge so thatacceleration of the scanning head does not apply a torque to the bridgethat would adversely effect the positions of the carriages at each end.6. Apparatus as claimed in claim 1 in which drive means and locationmeans are provided on the bridge to control longitudinal movement of thescanning head along the bridge.
 7. Apparatus as claimed in claim 6 inwhich the drive means comprises an electric linear motor and thelocation means comprises an optical grating.
 8. Apparatus as claimed inclaim 1 in which the scanning head preferably scans the light-beamrepeatedly across a short distance and is advanced between individualscans so that successive lines are scanned across the direction ofadvance of the scanning head along the bridge, the light-beam therebycovering a band or swathe in a single traverse across a sheet beingscanned.
 9. Apparatus as claimed in claim 8 in which successivetraverses of the scanning head are offset each by the width of a swathefrom the last swathe as the bridge is indexed across the bed so that thewhole of the sheet is scanned by the head.
 10. An apparatus as claimedin claim 1, in which the location means at each end of the bridgecomprises an optical sensor and a grating which move relative to oneanother, the sensor producing sine and cosine voltage output signalscorresponding to the spacing of lines ruled at regular intervals alongthe grating, the system further comprising a processor that processesthese signals to compensate for variations in the distance between thesensor and the grating, the processor serving to determine theinstantaneous value of the root of the sum of the squares of the outputsignals, and divide each of the output signals by this root-mean-squarevalue before processing the output signals to determine the position ofthe scanning head.
 11. An apparatus as claimed in claim 1, furthercomprising a store for photosensitive plates located beneath the bed,and feed means for feeding photosensitive plates from the store to thebed for scanning.
 12. An apparatus as claimed in claim 11 in which thestore is such as to store the plates upright in one or more cassettesbeneath the bed, and the feed means selects plates as required from thecassettes and feeds each to a feed station along one side of the bed,from where the plate is fed horizontally onto the bed.
 13. An apparatusas claimed in claim 12 in which each plate is fed edge first onto thebed with the leading edge generally normal to the feed direction.
 14. Anapparatus as claimed in claim 13 in which the plates are stored uprightin cassettes beneath the bed, and feed means selects plates and feedsthem to the feed station.
 15. An apparatus as claimed in claim 1,further comprising a plate feed station located along a first side ofthe bed to feed photosensitive plates one at a time onto the bed in afirst feed direction, and a film feed station located along a secondside of the bed adjacent said first side to feed photosensitive filmsheet onto the bed in a second feed direction transversely of said firstfeed direction.
 16. An apparatus as claimed in claim 15 in which thecassettes are loaded beneath the bed along the same side as the filmfeed station so that the handling of the photosensitive media takesplace in a common work area.
 17. An apparatus as claimed in claim 15 inwhich the plates and film sheet are removed from the bed in said secondfeed direction so that they can be further handled or processed in thesame work area.
 18. An apparatus as claimed in claim 1, furthercomprising sheet location means having air ducts adapted to supply alayer of air to the flat bed on which the sheet can float, and at leastone stop member located on the bed in relation to an inclination of thebed from the horizontal so that a sheet floating on the bed slides underthe action of gravitational force against the stop.
 19. Apparatus asclaimed in claim 18 in which stop members are located to supportadjacent lowest edges of said sheet so that it is located in mutuallyorthogonal directions on said bed.
 20. An apparatus as claimed in claim1, further comprising sheet pick up means mounted on the head to contactan upper surface of the sheet and hold it so that it can be moved by thehead to be located in position on the bed for subsequent scanning. 21.An apparatus as claimed in claim 1, further comprising location means toposition said sheet on said flat bed, holding means to hold said sheetin position on said flat bed, and punch means to punch register holes insaid sheet while it is held in position on said flat bed.
 22. Apparatusas claimed in claim 21 in which the punch means is moveable relative tothe bed so as to advance to punch the sheet and retract to allowscanning of said sheet.